Network connection was spotty, so tried to turn off Wi-Fi and switch to tethering. Wi-Fi stayed on. Successively more apps started to beachball. Tried quitting as many as possible, but in the end even the clock stopped updating so I decided to do a hard reset.

This is my first involuntary reboot on Yosemite, and this machine is doing well, approaching the best MTBF I've had so far (currently at 37.875 days, previous record was 41.86 days).